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March 28, 2011

Mega-Quakes Do Not Cause Quakes In Other Regions

by Sam Savage

Mega-quakes, such as the recent massive temblor off the coast of Japan are not expected to set off other large quakes in distant regions of the world, according to a study published by the US Geological Survey (USGS).

There is a current novel theory that a large quake in one continent can unleash a quake in another. Tom Parsons and Aaron Velasco of the US Geological Survey and the University of Texas at El Paso explained their counter theory to AFP.

Parsons and Velasco looked at all earthquakes around the world of magnitude 7.0 or above that were followed by quakes greater than magnitude 5.0. Their research counted 205 main shocks, and more than 20,000 hypothetical secondary shocks.

A "significant increase" in seismic activity in adjoining areas of the fault was found. This confirms large quakes place stress on nearby sections of the same fault, and like buttons on a shirt, pop off one by one.

Beyond this localized effect, however, which typically occurred two or three rupture lengths farther down the same fault, there was no increase in seismic risk. "The regional hazard of larger earthquakes is increased after a main shock, but the global hazard is not," concludes the study, which appears in the journal Nature Geoscience.

The theory of long distance earthquakes triggered by distant shifts in tectonic plates was released in a 2009 paper. This study noted in a big change in stress in a closely monitored section in California's San Andreas fault after the 9.1 quake that struck west of Sumatra in December 2004, 5,000 miles away.

After such a large quake, Parson related, "the whole planet is an aftershock zone." Pulses of seismic energy show up even in places such as Australia which are not quake-prone regions.

Surface waves from a quake travel along the earth's relatively fragile crust, which effectively channels their energy around the world. There was, however, no evidence that the pulse brings distant faults closer to rupture, Parsons said in an interview with AFP.

The authors of the 2009 paper "didn't actually see a large earthquake occurring" in California after the 2004 Sumatra quake, he noted. "There are clusters of events that happen, then there's periods of quiet, it's just the Earth behaving as it does -- there is variability," he said.

"There might also be some causes that we don't understand. But there's nothing that leaps out about this last decade that's unusual."

Seismologists are crunching data as fast as they can to calculate the potential increase in risks to faults south of where the Japan earthquake was centered, for it underlies Tokyo and caused devastation in 1923 and 1703, he added.

"There's certainly a concern. Every time you have a disturbance like this, the odds of other earthquakes are raised for a period of time. It can be decades."